The present invention relates generally to analog to digital converters (ADC), and, more particularly, to a pre-charge buffer for an ADC that pre-samples input signals.
Sampler circuits are widely used to sample input signals in signal processing applications such as in ADCs and digital-to-analog converters (DACs). A sampler circuit samples an input signal during a sample phase of a sample and hold cycle and holds the input signal during a hold phase of the sample and hold cycle.
Typically, the sampler circuit is used with an ADC to sample multiple input signals (i.e., channels). The sampler circuit includes multiple input channels for receiving the input signals, multiple input switches, and a sample and hold circuit. The sample and hold circuit is connected to the input channels by way of the input switches. The sample and hold circuit includes a sampling capacitor and a sampling switch. The sampling switch provides the input signals to the sampling capacitor when one of the corresponding input switches is active.
During the sample phase, the sampling capacitor receives an input signal via a corresponding input switch. The sampling capacitor is charged to a voltage level of the input signal, thereby sampling the input signal. During the hold phase, the sampling capacitor holds the input signal and provides it as a sampled output signal to the ADC, which then generates a corresponding digital sampled output signal.
In applications such as adaptive backlight illumination and battery voltage monitoring, which use the sampler circuit along with the ADC, the time required to charge the sampling capacitor to a voltage level of the input signal can be longer than the sample phase. Hence, the sampling capacitor may not be charged to the voltage level of the input signal during the sample phase, which can cause the sampling capacitor to provide an incorrect measure of the input signal to the ADC. The input channels corresponding to such input signals are referred to as “weak input channels.”
One way to overcome the weak channel problem is to use a pre-sampling circuit that samples the input signal before the sample phase. The pre-sampling circuit is connected to an electrode of the sampling capacitor, and pre-samples the input signal and then outputs the pre-sampled input signal during the sample phase. The sampler circuit samples the input signal after the pre-sampling and provides the sampled input signal to the sampling capacitor. Pre-sampling of the input signal allows the sampling capacitor sufficient time to provide a correct measure of the input signal.
However, the input and sampling switches are implemented using transistors and when such transistors are closed (ON), a channel region of the transistor injects charge into the source and drain regions (known as charge injection). This charge injection is propagated to the sampling capacitor, which can introduce an error in the input signal sampled by the sampling capacitor, and consequently the sampling capacitor may provide an incorrect measure of the input signal to the ADC.
Further, during the sample phase, the charge is transferred from the sampling capacitor to the input switches. Such charge transfer, referred to as “charge sharing”, may reduce the charge stored in the sampling capacitor, such that the sampling capacitor may provide an incorrect measure of the input signals to the ADC.
It would be advantageous to have a sampling circuit that pre-samples the input signals and reduces the effects of charge sharing and charge injection.